This second revision of a competitive renewal application is based on an in-depth analysis of the mechanism(s) of tumor protection in tolerant mice where protection is initiated by CD4 T cell immunity against MUC.1. At the present time there are no reports showing that CD4 T cell tolerance against MUC.1 can be broken by specific vaccination, nor that CD4 T cell immunity generates tumor protection. Therefore, our model is new. Our studies are based on the induction of CD4 T cell responses against a sub-immunogenic T cell determinant of MUC,1 that exploits a new method to "jump-start" weak Th cell responses, Th-Th cooperation. The method of immunization is also new lymphocytes transgenic for a gene engineered with the two Th cell determinants involved in Th-Th cooperation. Working in MUC.1 transgenic mice during the past two and half years we have been able to show that Th-Th cooperation combined with transgenic lymphocyte immunization (TLI) effectively breaks self tolerance and induce protective immunity. In the experiments proposed in this competitive renewal our goal is to elucidate and understand the mechanism(s) of tumor protection in vivo. Specifically, we will revisit the role of MUC.1 reactive CD4 T cell and assess a potential contribution of CD8 T cells (Aim 1), including the role of central memory and effective memory CD4 and CD8 T cells (Aim 2). In a systematic step-wise approach, we will also study the contribution of bone marrow-derived dendritic cells (DC) and plamacytoid DC (pDC) (Aim 3), and see if activation of NK and NKT cells is part of the overall mechanism of protection (Aim 4). It is hoped that these studies will shed light on the relationship between immunity and protection, and will establish the ground rules for successful intervention against cancer expressing MUC.1 in humans. [unreadable] [unreadable]